Infectious Diseases Affecting the Skin and Eyes

The Skin and Its Defenses

  • Learning Outcomes:

    • Anatomy of Skin: Understanding its structure and defenses.

General Structure

  • Surface Area: Approximately 1.5 to 2 square meters.

  • Thickness: Ranges from 1.5 mm to 4 mm.

  • Components:

    • Hair

    • Nails

    • Sweat and Oil Glands

Epidermis

  • Stratum Corneum:

    • Keratin: Provides resistance to damage and abrasion.

    • Lipids: Provide water-repellent properties.

    • Regeneration: The entire epidermis is replaced every 25 to 45 days.

  • Stratum Basale:

    • Source of all epidermal cells; contains stem cells used for skin culture in burn victims.

Dermis

  • Composition: Made of connective tissue, rich in fibroblast cells and collagen fibers.

  • Functions:

    • Contains macrophages and mast cells.

    • Supports a dense network of nerves, blood vessels, and lymphatics.

    • Damage can lead to bleeding and blistering (due to separation of dermis and epidermis).

Follicles and Glands

  • Hair Follicles: Roots found in the dermis; associated with sebaceous and scent glands, which are sites for infections.

  • Sweat Glands: Separate from follicles, located in the dermis.

  • All glands have openings that facilitate passage through the epidermis, usually harboring a non-pathogenic microbiome.

Antimicrobial Defenses

  • Antimicrobial Peptides:

    • Disrupt bacterial membranes; examples include LL-37.

    • Help maintain low microbial counts on the skin.

  • Sebum:

    • Secreted by sebaceous glands to lower skin pH, inhibiting microorganisms.

    • Lipids serve as nutrients for normal microbiota.

  • Sweat and Lysozyme:

    • Sweat maintains low pH and high salt concentration.

    • Lysozyme found in bodily fluids like sweat and tears; breaks down peptidoglycan in bacterial cell walls.

Normal Biota of the Skin

  • General Characteristics:

    • Skin is inhospitable to many microbes due to dryness, salinity, and antimicrobial lipids.

    • Densest microbial populations are found in moist areas (e.g., skin folds).

  • Human Microbiome Project Findings:

    • Hundreds of microbial species exist on the skin.

    • Staphylococcus epidermidis and Propionibacterium acnes are predominant, with about 4% of the population carrying Staphylococcus aureus.

Key Microbes in Skin Biota

  • Bacteria:

    • Staphylococcus epidermidis

    • Propionibacterium

    • Corynebacterium

    • Lactobacillus

    • Bacteroides

    • Others present include non-hemolytic streptococci.

  • Yeasts:

    • Malassezia and Candida species.

Important Biota Characteristics

  • Defenses:

    • Keratinized surfaces, sloughing, low pH, high salt, lysozyme.

Skin Diseases Caused by Microorganisms

  • Common Conditions:

    • MRSA (Methicillin-resistant Staphylococcus aureus)

    • Impetigo: caused by S. aureus and S. pyogenes.

    • Cellulitis, scalded skin syndrome, gas gangrene, various rashes and mycoses.

Specific Pathogens

  • Staphylococcus aureus:

    • Gram-positive; forms clusters and withstands environmental extremes. Produces exfoliative toxins leading to skin damage in conditions like scalded skin syndrome.

  • Streptococcus pyogenes:

    • Causes strep throat and skin infections like impetigo and necrotizing fasciitis. Produces M protein facilitating adherence and evading immune response.

  • Clostridium perfringens:

    • Causes gas gangrene; produces toxins that damage tissues.

Other Notable Conditions

  • Vesicular or Pustular Rash Diseases:

    • Chickenpox, smallpox, and hand, foot, and mouth disease, each caused by specific viruses (e.g., human herpesvirus 3, variola virus).

  • Measles and Rubella:

    • Caused by respective viruses, with unique symptoms and significant epidemiological impact.

Eye and Its Defenses

  • Components & Functions:

    • Conjunctiva: Secretes protection and lubrication for the eye.

    • Cornea: Protective dome that can quickly regenerate.

  • Tears: Crucial for cleaning and lubricating the eye surface; contain lysozyme and lactoferrin for microbial defense.

  • Reduced Immune Response (Immune Privilege): Protects vision but makes the eye susceptible to infections.

Ocular Infections

  • Common Eye Diseases:

    • Bacterial conjunctivitis, viral conjunctivitis, keratitis, and conditions caused by Chlamydia trachomatis.

  • Unique Considerations:

    • Neonatal conjunctivitis often requires different diagnostic and treatment approaches compared to adults.

Conclusion

  • Understanding the components, defenses, and common pathogens affecting the skin and eyes is critical for microbiology and medical studies. Effective prevention and treatment strategies stem from a deep comprehension of these subjects.

The skin has a surface area of 1.5 to 2 square meters and thickness varying from 1.5 mm to 4 mm, consisting of components such as hair, nails, and glands. The epidermis includes the stratum corneum, which provides damage resistance, and the stratum basale, which generates new skin cells. The dermis, made of connective tissue, contains nerves, blood vessels, and cells that protect against infections. Antimicrobial defenses like sebum and lysozyme maintain low microbial counts. The skin hosts various microorganisms, with notable species being Staphylococcus epidermidis and Propionibacterium acnes. Common skin diseases stem from pathogens like Staphylococcus aureus and Streptococcus pyogenes, leading to conditions such as MRSA and impetigo. The eye, protected by the conjunctiva and cornea, can also be prone to infections. Understanding these defenses and pathogens is crucial for medical studies.